Numerical simulation was used to research the two-phase flow in the axisymmetric inlet square separator, including the basic flow characteristics, separation efficiency and comparing the difference between single-inlet and double-inlet separator. The effect of the reflecting cone’s geometrical sizes on pressure loss and efficiency was discussed. And the wall erosion condition because of solid particle impingement was calculated and analyzed. The tangential inlet diffuse square separator using RSM modeling that considers anisotropic situation was researched. The distribution of axial, tangential and radial velocity and the pressure loss calculated were analyzed. The results show that there is the typical double deck flow structure in the separator, the square section influences the tangential velocity in the corner, the pressure distribution divides in the inlet on the reflecting cone, and the pressure loss increases with the increase of inlet velocity.